


Classical Cryptanalysis

by BlackEyedGirl



Category: Sherlock (TV)
Genre: Comment Fic, Community: fic_promptly, Gen, Siblings
Language: English
Status: Completed
Published: 2012-02-02
Updated: 2012-02-02
Packaged: 2017-10-31 07:19:38
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 497
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/341424
Author URL: https://archiveofourown.org/users/BlackEyedGirl/pseuds/BlackEyedGirl
Summary: <blockquote class="userstuff">
              <p>Mycroft finds Sherlock's note where his credit card should be. [Written for a fic_promptly prompt. Spoilers for The Reichenbach Fall]</p>
            </blockquote>





	Classical Cryptanalysis

Eight months after Sherlock’s funeral, Mycroft opens his wallet to extract his Visa card and finds a folded note instead.

All it says is: _Needed additional funds for case. SH_

Mycroft presses his palms flat around the paper.

Anthea watches him for a moment and then asks, “Sir?” He passes over the American Express in its place and hears the beeps of the PIN machine as she pays for both of them.

John would say that it is exactly like Sherlock to do that – to act as though nothing had happened. (John is saying nothing much to anyone right now.) But this is not like Sherlock at all. Sherlock wouldn’t leave a note for Mycroft. When Sherlock made off with Mycroft’s things, he didn’t point it out – he had always assumed that Mycroft should be clever enough to know. Even when they were children, Sherlock would steal books and rock samples and LEGO blocks – if Mycroft didn’t notice and hunt them down again himself, he didn’t deserve to have them. 

Mycroft calls his bank. “I need to check recent activity on my card.” He listens while a calm-voiced woman recites a reverse record of his whereabouts the previous evening. There is nothing he doesn’t recognise. There is still time to verify this. He says, “Then I will also need to change the PIN number on the card.”

They had written each other notes, when they were children. When Mycroft went away to school, he sent Sherlock letters. They used ciphers, not terribly complicated, since that wasn’t the point. The point was to see how quickly they could figure out the deciphering word without simply resorting to crude frequency analysis. The only rule was that there had to be a plausible connection between the current and previous word. Sherlock was always good at finding links. 

Mycroft goes through the computerised process of changing the number. 

Four hours later he calls the bank again. There are a series of cash withdrawals Mycroft did not make. The PIN was entered incorrectly once in the process of the first withdrawal.

The last letter was Mycroft’s to Sherlock, sent the January after Sherlock had taken a deep breath and said, ‘ _Please_ don’t go back to school without me.’ Mycroft had noticed, those last months before the jump, that Sherlock had recently rediscovered the power of that particular word, although he still wouldn’t use it with Mycroft. After all, it hadn’t worked before. Mycroft had never known if Sherlock had deciphered the last letter, or if he had destroyed it in a fit of pique. He hadn’t mentioned the contents, but without knowing that Mycroft’s code had jumped from Astronomy to History, what of today?

Mycroft can picture Sherlock, gesturing with one dismissive hand, brushing off the ease with which he deciphered his brother’s code: _1840, the Treaty of Waitangi, Mycroft, really?_

Mycroft interlocks his fingers and, alone in the darkness of his office, presses them tightly to his mouth.

Sherlock is alive.


End file.
